Apparatus for starting charged particle accelerators



March 14, 1950 Filed July 6, 1949 A. M. GUREWITSCH 2,500,749 APPARATUS FOR STARTING CHARGED PARTICLE ACCELERATORS 2 Sheets-Sheet 1 Inventor": Anajcoie' M .G urewitsch,

. H's At COPTIey GUREWITSCH 2,500,749

- A. M. APPARATUS FOR STARTING CHARGED PARTICLE ACCELERATORS March 14, 1950 Filed July 6, 1949' 2 Sheets-Sheet 2 Fig.4. A

A.C. 55 SOURCE A PHA SHIF A AA v V v Inventor: Anatole MGurewitsch,

His Attorney Fig 7.

Patented Mar. 14, 1950 APPARATUS FOR STARTING CHARGED PARTICLE ACCELERATQRS Anatole M. Gurewitsch, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application July 6,1949, SerialNo. 103,311

15 Claims. (01. 250-27) The presentinvention relates to apparatus. for accelerating charged particles and in particular is Concerned with improved means-for starting such apparatus.

This application is a continuation in-part of my copending applicationSerial No. 788,217, filed November 26, 1947, now abandoned.

An apparatus in which 'chargewparticles are accelerated in an orbital path "by a time-varying magnetic field is commonly "known as a betatron, andthe operation of'such apparatus is described in the Journalof Applied Physics, vol. 16, (1945). Anapparatus in'which charged particles are 'accelerated'in an orbital path by" successive applications of an electric'field which maintains stability "oftheparticles in time is commonly known as' a synchrotron, and the operation of such apparatus is described by E.

M. McMillan in the Physical Review, vol; 68,-

page 143.

An article appearing, in the Journal of Applied Physics,vol. 18, No. 9 (1947l-describ'es ap'-' paratus, also sometimes referred to as a synchrotron, whereby charged particles are ac-' celerated in an orbital path both by a timevarying magnetic field and successive applications of an electric field. The initial acceleration of the charged particles is caused by the time-varying magnetic field= as in the betatron. When the charged particles have been accelerated to a predetermined velocity in this manner, localized cyclically-varying electric'field act upon the gyrating particles to continue their acceleration.

In a'betatron, as described'forexample in Westendorp U. S; Patent 2;394,0'71"issued February 5, 1946," the magnetic accelerating field is generated by one or more windings which are placed upon'amagnetic core and energized by alternating current:' In'orde'r that the apparent power supplied to these windings maybe mini- Such is not always the case with the synchrotron, especially during an initial starting period which may be of many cycles duration, and as a result of which large substantially non-resonant currents may flow with consequent overloading of the power supply system. The reason is that in a synchrotron the instantaneous inductance of the field windings is not constant, i. e., not linear, but varies cyclically with the energizing voltage by virtue ofthe 'fact that a portion of the magnetic circuit, i. e. a central core, saturates early in the cycle of energizing voltage and remains saturated until that voltage returns to a value near zero /120 of a second later for 60 cycle operation). Because of this situation, it has been customary to compromise by maintaining resonance and hence minimized power only during the normal'steadystate operating condition byresonating the aforementioned capacitors with the so-oalled average inductance of the windings. However, with this arrangement, during the initial starting period before the apparatus reaches steady state operation and when the central core is in an unsaturated condition, the winding circuit is not resonated; consequently, when the winding circuit is energized for starting, large leading currents will be drawn from the sup ly mains before the operating voltage is reached and an overloading the size of the alternating current power supply required for starting the accelerator may be much smaller than previously required, and 'th-is- -,These and other features of this invention willbe discussed more fully in connection with the accompanying drawings, in which Fig. l is a vertical section of an accelerator which may be provided with a starting auxiliary embodying this invention; Fig. 2 is a top view of the accelerator shown partly in section; Figs. 3 and 3a are simplified schematics useful in explaining electric circuit phenomena utilized in this invention; Fig. i is a schematic representation oi?v a preferred circuit for an accelerator provided with a starting auxiliary embodying this invention and Figs. 5, 6 and "7 are graphical representations helpful in explaining the invention. In view of the similarity of figures throughout 3 the drawings, like numerals have been used to designate like parts throughout.

An annular path along which electrons are accelerated is defined by a rotationally symmetrical container l shown in Figs. 1 and 2. Enclosed within the container, which ordinarily consists of glass, is an electron gun Ii which supplies electrons to be accelerated. The electron gun is energized by conductors I2 which are sealed into a glass side arm l3. In order that the electrons may be accelerated without suffering excessive collisions with gas molecules, the container ll! is highly evacuated, and suitable conductive coatings M and [5 of a material such as silver are deposited upon its interior surface to provide means for establishing high frequency electric fields within the container. The coatings are separated by gaps l6 and I! which are substantially diametrically opposed to each other within the annular space defined by the container and across these gaps localized electric fields appear when the coatings are energized by a high frequency source in a manner hereinafter described. Toreduce the circulation of induced electric currents, the coatings are longitudinally subdivided.

The magnetic structure which provides a path for the various fluxes required for successful operation of the apparatus comprises a laminated rectangular magnetic frame structure it and laminated pole pieces l9 and 29. A centrally located laminated core member 2! aligned with the axis of the container 10 is separated from the remaining magnet structure by insulating spacers 22 and 22', the thickness of which determine the diameter of the electron orbit during betatron acceleration. Windings 23 and 24 serve as power input windings of the excitation circuit and are energized from a source of alternating current (not shown). The magnetizing windlugs 25 and 26 are located between the input windings 23 and 24 near pole pieces l9 and 20 respectively and may be excited by the mutual flux established by the input windings.

Connected by leads 28 to the secondary windings, which are themselves connected in series, is a capacitor 21 which is used to resonate the magnetizing circuit at the desired operating frequency of the apparatus, for example, at 60 cycles per second. It is to be understood that capacitor 21 may in practice consist of a bank of capacitors of suitable rating and number and that the representation here is of the total capacity of such a bank.

As hereinbefore stated large leading currents will be drawn from the alternating current supply mains when such synchrotron apparatus is started, because the magnetizing winding circuit, including windings 25, 26 and capacitor 21 is not resonated at the starting condition when core 2! is unsaturated. This invention proposes to eliminate such difficulties by resort to elec-, trical circuit phenomena which may be best explained by reference to Fig. 3. Fig. 3- shows an iron cored inductor L connected in parallel with a capacitor C, and a battery B connected in parallel with capacitor C. Switch Si is in series with the LC combination and switch S2 is in series with the battery B. If S2 is closed and S1 opened, battery B will charge capacitor C to a potential corresponding to its own terminal volt- Then if S2 is opened and S1 is closed, the energy stored in capacitor C by battery 13 will transfer to inductor L at a rate dependent upon the relative values of the circuit parameters, L

and C. This energy transfer will occur periodically with the energy being stored first in the capacitor and then in the inductor, i. e. the circuit will oscillate at its natural frequency or be resonant at a frequency dependent upon the relative values of the circuit parameters. Resistance, dielectric and iron losses in the circuit will merely cause the amplitude of the oscillations to decrease exponentially with time but will not alter their frequency.

If the magnetic core of the inductor saturates at a particular value of current through the windings, the inductance of the inductor will become non-linear, and the natural frequency of oscillation of the circuit under steady state conditions will be determined by the so-called average" inductance in combination with the capacitance. However, to initiate oscillations of an amplitude which exceeds the saturation current of the inductor and simultaneously obtain oscillations of a desired frequency, it is necessary only to choose the size of capacitor C such that it will cause resonance with the average inductance of inductor L at that frequency and to charge the capacitor C to a voltage approximatm ly equal to the amplitude of the oscillations desired.

The utilization of such phenomena in connection with synchrotron apparatus as described in conjunction with Figs. 1 and 2 may be explained by reference to the simplified circuit in Fig. 3a. The circuit is similar to Fig. 3 except that magnetizing windings 25, 26 (Fig. 1) replace inductor L (Fig. 3), capacitor bank 2'! (Fig. 1) replaces capacitor C, and input windings 23, 24 (Fig. 1), along with switch S3 in series with an alternating current source, have been added. Capacitor 2'! is charged to the proper potential after Sz has been closed. S2 is then opened and S1 is closed and the circuit including windings 25, 2G and capacitor 2! begins to oscillate. Since the primary windings 23, 24 are on the same core as the secondary windings 25, 26, a voltage is induced in them by the oscillations in the secondary. Switch S3 must be closed at the proper time to connect the alternating current source to the primary windings when the voltages are nearly in phase. The alternating current source supplies enough power to the oscillatory circuit to compensate for I R and other losses and thus the oscillations are sustained. It will be apparent to one skilled in the art that capacitor 2'! may be considered as resonating with the inductance of the magnetizing windings which are closely coupled to the power input windings, thereby in effect creating a parallel resonant circuit whose losses are supplied by the alternating current source.

In Fig. 4 is shown a circuit for the practical adaptation of the hereinbefore described phenomena to the starting of synchrotron ap aratus. Coils 23, 24 are energized by an alternating current source 29 through a voltage regulator 30 and a circuit breaker 3| having closure coils 55. The series connected windings 25, 28 are connected by leads 28 to resonating capacitor 21 whose function has been described previously by reference to Figs. 1 and 3a. To provide a means of interrupting the circuit including windings 25, 26 and capacitor 21, a triggered spark switch 3'. comprising a first electrode 32, a second electrode 33 and a triggering electrode 34 may be employed. A resistor 35 of suitable size serves to maintain the electrode 34 at a desired potential with respect to electrode 32. Means for by passing switch 31 may be provided by a circuit breaker 35 having closure coils 56.

For the purpose of charging capacitor 21 a direct current rectifying circuit, comprising an isolating switch 38, a current limiting resistor 38, a high vacuum diode rectifier tube 39, a supply transformer 40 and a switch 4| may be provided. The circuit is energized from the alternating current source 29 through leads 42, and its output is placed across capacitor 21 through leads 43.

v To insure that the closing of spark gap switch 31 is properly synchronized with the alternating current source, a circuit criterion which will be more fully explained later in connection with Figs. 5, 6 and 7, a phase shifter 44, which is energized from the alternating current source, may be employed. The output of the phase shifter energizes a peak ng transformer 45. One side of the secondary winding of the peaking transformer is connected to ground while the other side feeds the grid 49 of a grid controlled gaseous discharge tube 41 whose cathode 48 is grounded; Power is supplied to the tube circuit in a conventional manner from a direct current supply (not shown but indicated as +B) through a decoupling resistor 50 connected to the tube plate circuit. The tube plate circuit includes the primary windings of a step-up transformer 52, a capacitor 53, and starting switch 46. One side of the secondary winding of transformer 52 is connected to windings 25 and 26 and to ground (considering windings 25 and 26 to be electrically similar, then the connection is a center tap between windings 25 and 26); the other side of the magnetizing winding is connected to triggering electrode 34 through a capacitor 54.

When it is desired to start the synchrotron apparatus c rcuit breakers 3| and 36 are placed in their open positions as is starting switch 46. Capacitor 21 is charged, by closing switches 4| and 38, to approximately operating voltage, i. e. the voltage which corresponds to the instantaneous peak alternating voltage that is induced in secondary windings 25, 26 when power is applied to input windings 23, 24. After capacitor 21 has been charged, starting switch 46 is closed, thereby energizing the plate circuit of tube 41. A voltage pulse transmitted from peaking transformer 45 to the grid 49 fires tube 41, i. e. causes it to become conducting; and capacitor 53, which has been charged to +B potential through the plate voltage supply, discharges through the primary winding of transformer 52 and tube 41. The discharge through the primary winding induces a voltage in the secondary winding of transformer 52 and, since one side of the secondary is maintained at ground potential, this induced voltage changes the potential of triggering electrode 34, with res ect to ground and, hence, electrode 33. The change in potential of electrode 34 causes a breakdown between it and electrode 33, and the resulting small arc drop brings electrode 34 to nearly the same potential as electrode 33. Con- 'sequently, because the potential difference between electrodes 34 and 32 has been increased and because ions are generated by the arc, the arc is extended to electrode 32. It should be noted that to initiate the arc the polarity of the induced voltage in the secondary winding of transformer 52 should be negative with respect to ground; because if electrode 33 has been charged to a high positive potential above ground, then the potential difference between electrode 34 and 33 can only be increased by driving the triggering electrode below ground potential. I

' By this sequence of events spark gap switch 31 is closed, and capacitor 21, which has been charged to a desired potential, is placed in circuit with accelerator windings 25, 26. The periodic transfer of energy described in conjunction with Figs. 3 and 3a now occurs, and the circuit begins to oscillate. The oscillatory voltage appearing across windings 25, 26 induces a voltage across windings 23, 24 which in turn energizes the closure coils 55 and 56 of circuit breakers 3| and 36, respectively. When circuit breaker 3| closes, the alternating voltage source is connected in proper time phase to the primary windings 23, 24 of the synchrotron, and the apparatus has been started. Circuit breaker 36 is provided merely to shunt spark gap switch 31 in order that the arc may be extinguished after the circuit has been made between windings 25, 26 and capacitor 21.

The desirability for arranging that spark gap switch 31 to be closed in proper time phase with alternating current source 29, may best be explained by reference to Figs. 5, 6 and '7. The voltage of source 29 is conventionally represented in Fig. 5 by a constant amplitude and constant frequency (e. g. 60 cycles) sine wave. The voltage induced in windings 23, 24 by the oscillations in windings 25, 26 (initiated by the closure of switch 31 as hereinbei'ore explained) is represented in Fig. 6. The voltage induced in windings 23, 24 is shown as being initially nearly equal in amplitude to the amplitude of the source voltage, a result which may be obtained by the proper charging of capacitor 21 before switch 31 is closed in the manner explained previously. Since power losses must appear in practical circuits, the amplitude of the oscillations in the circuit including windings 25, 26 and capacitor 21 will decay exponentially with time, and hence the voltage induced in windings 23, 24 will decrease in amplitude somewhat as indicated. The frequency of the induced voltage will be approximately equal to the source frequency for reasons explained in connection with Fig. 3.

If switch 31 is closed at an arbitrary time, the initiated oscillations may have any time phase relation with respect to the source voltage, e. g. leading by a time h as indicated. Under these conditions if circuit breaker 3| is closed, troublesome transient voltages and currents would be generated in the source and the synchrotron Wind ng circuits. To synchronize properly the induced voltage with the source voltage, phase shifter 44 is adjusted to advance or retard (de pending upon circuit parameters) the pulse transmitted from peaking transformer 45 so that switch 31 closes at a time when the two voltages will be in phase as shown in Fig. '1. Circuit breaker 3| may then be closed shortly thereafter without causing any circuit disturbances. The exponential decay of the induced voltage as shown in Figs. 6 and 1 is greatly exaggerated for the sake of clarity and in practice the closing of circuit breaker 3| could be delayed for several cycles before any appreciable difference in amplitude between the induced oscillation voltage and the source voltage would obtain.

The additional circuits required to place the apparatus in full operation are indicated conventionally in Fig. 4. As described in Westendorp U. S. Patent 2,394,071 the magnetic peaking strip 60 changes cyclically and abruptly from a saturated to an unsaturated condition in accordance with the variation of magnetic field, thereby controlling the injection of electrons. When the switch 6| is closed the variation of voltage in the winding of the peaking strip 60 through the intermediary of a control device 62 energizes a pulse generator 63 which feeds an electron gun ll (Fig. 1), whereby electrons are introduced into the time-varying magnetic field generated by windings 25 and 26. As conventionally indicated in Fi 4 the electron gun comprises a thermionic filament 64, a focusing cup 65 and a housing 66. After a predetermined time interval during which the electrons are accelerated solely by the magnetic field. a delayed action switching device 61 connects a high frequency source 68 to the accelerating electrodes i4, 15 on the interior of the accelerating chamber as diagrammatically indicated in Fig. a. When a predetermined period of acceleration has elapsed, a second electronic delay device 69 effects a discharge of the accelerated electrons; for example, upon a target 10 (Fig. 1).

Although a spark gap type switch 3'! (Fig. 4) has been employed as a means for completing the aforesaid resonant circuit, this invention is not limited to that specific type of switch. Any type of circuit interrupter capable of being closed within a specified time, e. g. a circuit breaker, may be employed to advantage. Nor is the power output of the direct current supply which is used to charge capacitor bank 21 a critical consideration, because the time required to charge the capacitor bank does not afiect the operation of the circuit. Switch 38 (Fig. 4) need not be opened at any particular time in the sequence of events and therefore may be a manually operated switch of proper ratin In practice the proper total capacity of capacitor bank 21 (Figs. 3a, 4) may be selected by adding or removing capacitors until maximum oscillatory current flows in the secondary winding circuit when the apparatus is in operation.

While the invention has been described by reference to particular embodiments thereof, it will be understood that numerous changes may be made without actually departing from the invention. In particular, although the description has been concerned mainly with the acceleration of electrons, it is obvious that the invention is equally applicable in connection with other charged particles, such as positive or negative ions. I, therefore, aim in the appended claims to cover this and all such equivalent variations of application and structure as are within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Apparatus for the acceleration of charged particles including a magnetic core having energizing windings magnetically coupled thereto, a source of alternating voltage for energizing said windings, said core being magnetically saturable during a portion of the cycle of said voltage when said windings are energized during normal operation of said apparatus, a capacitance for resonating said windings at substantially the frequency of said voltage when said windings are so enercircuit opening and closing means for conpectin- 3 said capacitance to said windings, a source of voltage and means for connecting the same to said capacitance prior to the connection of said capacitance to said windings whereby said capacitance may be precharged to approximately nor-- mal operating voltage before the application of said alternating voltage to said windings.

2. Apparatus for the. acceleration of charged particles including a magnetic core having energizing windings magnetically coupled thereto, a

source of alternating voltage for energizing said windings, means for connecting said source to said windings, said core being magnetically saturable during a portion of the cycle of said voltage when said windings are energized during normal operation of said apparatus, a capacitance for resonating said windings at substantially the frequency of said voltage when said windings are so energized, a source of unidirectional voltage and means for connecting the same to said capacitance prior to the connection of said capacitance to said windings and prior to the con nection of said source to said windings whereby said capacitance may be precharged to a unidirectional voltage approximately equal to the instantaneous peak value of the normal alternating operating voltage, and circuit opening and closing means for connecting said capacitance in circuit with said windings after precharging and prior to the connection of said source 01' alternating voltage to said windings whereby oscillations at the frequency of said source of alternating voltage are set up in said circuit.

3. Apparatus for the acceleration of charged particles by both a time varying magnetic field and successive applications of electric field including a magnetic core having energizing windings magnetically coupled thereto, a source of alternating voltage for energizing said windings, switch means for connecting said source to said windings, said core being magnetically saturable during a portion. of the cycle of said voltage when said windings are energized during normal operation 01 said apparatus, a capacitance for resohating said windings at substantially the frequency of said voltage when said windings are so energized, a source of unidirectional voltage and means for connecting the same to said capacitance prior to the connection of said capacitance to said windings and prior to the connection of said source to said windings whereby said capacitance may be precharged to a unidirectional voltage approximately equal to the instantaneous peak value of its normal operating voltage, circuit opening and closing means for connecting said capacitance in parallel circuit with said windings after precharging and prior to the connection of said source of alternating voltage to said windings whereby oscillations at the frequency of said source of alternating voltage are set up in said parallel circuit, means operable to close said circuit opening and closing means, a controllable phase shifter connected to said source of alternating voltage and adapted to energize said last mentioned means at such time that the phase of said oscillations will be matched to the phase of said source of alternating voltage, and means responsive to said oscillations for actuating said switch means for connecting said source of alternating voltage to said windings.

4. Apparatus for the acceleration of charged particles by both a time varying magnetic field and successive applications of an electric field including a magnetic core having energizing windings magnetically coupled thereto, a source of alternating voltage for energizing said windings, switch means for connecting said source to said windings, said. core being magnetically saturable during a portion of the cycle of said voltage when said windings are energized during normal operation of said apparatus, a capacitance or resonating said windings at substantially the frequency of said voltage when said windings are so energized, a source of unidirectional voltage and means for connecting the same to said capacitance prior to the connection of said capacitance to said windings and prior to the connection of said source to said windings whereby said capacitance may be precharged to a unidirectional voltage approximately equal to the instantaneous peak value of its normal operating voltage, a spark gap for connecting said capacitance in parallel circuit with said windings after precharging and prior to the connection of said source of alternating voltage to said windings whereby oscillations at the frequency of said source of alternating voltage are set up in said parallel circuit, a triggering electrode in said gap for inducing current flow thereacross, a circuit for applying voltage pulses to said triggering electrode, a controllable phase shifter connected to said source of alternating voltage and adapted to energize said circuit to apply to said triggering electrode pulses at such time that the phase of said oscillations will be matched to the phase of said source of alternating voltage, and switch means responsive to said oscillations for actuating said first mentioned switch means for connecting said source of alternating voltage to said windings.

5. Apparatus as in claim 4 in which said circuit for applying voltage pulses comprises a gaseous discharge tube including a cathode, an anode and a control grid, a peaking transformer having a secondary winding connected to said grid and a primary winding energized by said phase shifter, and a circuit connected to said anode and to said triggering electrode whereby timed voltage pulses applied to said grid in predetermined phase relation to the phase of said alternating voltage eiiect corresponding voltage pulses applied to said triggering electrode.

6. Apparatus for the acceleration of charged particles including a magnetic core having energizing windings magnetically coupled thereto, a source of alternating voltage for energizing said windings, said core being magnetically saturable during a portion of the cycle of said voltage when said windings are energized during normal operation of said apparatus, a capacitance for resonating said windings at substantially the frequency of said voltage when said windings are so energized, circuit opening and closing means for connecting said capacitance to said windings, a source of voltage and means for connecting the same to said capacitance prior to the connection of said capacitance to said windings whereby said capacitance may be precharged to approximately normal operating voltage before the application of said alternating voltage to said windings, and actuating means for closing said circuit opening and closing means in time phase with said source of alternating voltage.

'7. Apparatus as in claim 6 in which said source of voltage is unidirectional.

8. Apparatus as in claim 6 including means responsive to the connection of said capacitance to said windings for connecting said source of alternating voltage to said windings.

9. Apparatus as in claim 6 including means for shunting said circuit opening and closing means.

10. Apparatus as in claim 8 including means additionally responsive to the connection of said capacitance to said windings, for shunting said circuit opening and closing means.

11. Apparatus for the acceleration of charged particles including a magnetic core having energizing windings magnetically coupled thereto, a source of alternating voltage for energizing said windings, means for connecting said source to said windings, said core being magnetically saturable during a portion of the cycle of said voltage when said windings are energized during normal operation of said apparatus, a capacitance for resonating said windings at substantially the frequency of said voltage when said windings are so energized, a source of unidirectional voltage and means for connecting the same to said capacitance prior to the connection of said capacitance to said windings and prior to the connection of said source to said windings whereby said capacitance may be precharged to a unidirectional voltage approximately equal to the instantaneous peak value of the normal operating voltage, and circuit opening and closing means operable by timed actuating means for connecting said capacitance in circuit with said windings after precharging and prior to the connection of said source of alternating voltage to said windings to set up oscillations at the frequency of and in phase with said source of alternating voltage.

12. Apparatus as in claim 11 in which said timed actuating means comprises a controllable phase shifter connected to said source of alternating voltage.

13. Apparatus as in claim 11 including means responsive to said oscillations for actuating said means for connecting said source of alternating voltage to said windings.

14. Apparatus as in claim 11 including means for shunting said circuit opening and closing means.

15. Apparatus as in claim 13 including means additionally responsive to said oscillations for shunting said circuit opening and closing means.

ANATOLE M. GUREWITSCH.

No references cited. 

